The technique for reducing the torque ripple of a motor has conventionally been proposed. For instance, various devices are disclosed in the following patent documents.
Japanese Patent No. 3028669 discloses a technique for providing skews in a rotor, and Japanese Patent Application Laid-Open No. 2001-8417 discloses a technique for providing skews in a stator. In terms of productivity, however, providing skews is not desirable.
Japanese Patent Application Laid-Open No. 11-98731 focuses attention to the configuration of magnetic flux barrier present between magnetic poles of a rotor, and intends to reduce the torque ripple by setting the angle it makes at a predetermined value or at an unequal pitch.
Japanese Patent Application Laid-Open No. 2000-217287 illustrates a technique for varying the thickness of a bridge present between a magnetic barrier of a rotor and an outer periphery, Japanese Patent Application Laid-Open No. 2002-44888 illustrates a technique for setting the angle of magnetic poles of a rotor within a predetermined range, and Japanese Patent Application Laid-Open No. 2004-180460 illustrates a technique for setting the angle of tip of a magnetic barrier of a rotor within a predetermined range.
However, employing an unequal pitch as in Japanese Patent Application Laid-Open No. 11-98731 absolutely requires of the rotor to have an asymmetric configuration in the circumferential direction relative to the border of magnetic poles. This causes magnetic fluxes to flow in a different manner between the magnetic poles, which is not desirable in terms of occurrence of vibrations and noise.
In Japanese Patent Application Laid-Open No. 2000-217287, since the thickness of a bridge is thin in the vicinity of the border between the magnetic poles, the difference in thickness of the magnetic member in the radial direction between the border of the magnetic poles and the center of the magnetic poles is not reduced, which does not lead to sufficient reduction of torque ripple. In addition, a mode of depressing the outer surface of the rotor is also illustrated, however, this configuration is not desirable in terms of occurrence of windage loss or wind noise.
The techniques introduced in Japanese Patent Application Laid-Open Nos. 2002-44888 and 2004-180460 do not absolutely require the asymmetry of configuration of the rotor or do not need to depress the outer surface, however, the difference in thickness of the magnetic member in the radial direction between the border of the magnetic poles of the rotor and the center of magnetic poles is not reduced, which still does not lead to sufficient reduction of torque ripple.